Recently we began wave soldering assemblies that have gold plated holes that are used for jumper wires. After wave soldering, there are often portions of the gold plated holes and pads that still have exposed gold. I'm not sure if it's necessary, but we touch up these gold areas by hand soldering. What is the best way to insure we get full solder coverage on these gold plated holes and pads?

C.W.

Experts Comments

There's a
couple things here.

First, exposed
surface finish is not a defect condition according to IPC-A-610E. Section 5.2.1
states that an acceptable connection can have "Exposed surface finishes that
are not part of the required solder fillet area."

As long as the
solder connection has the correct amount of hole fill (75% hole fill in most
cases), solder wetting around the wire and barrel of the hole on the solder
source side (270 degrees class 2, and 330 degrees class 3), and the proper
amount of wetting on the solder destination side (180 degrees class 2, and 270
degrees class 3) you do not need to touch the connection again.

Reheating a
connection multiple times when not required could heat damage the connection or
increase the thickness of the brittle intermetallics among other things.

The other question I
have is, are you removing the gold first? If not, you might be setting yourself
up for failures due to gold embrittlement.

Kris RobersonManager of Assembly TechnologyIPCKris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

Wave soldering requires careful optimization of several
variables to insure adequate hole-fill.
Is the flux you are using active enough, is the contact time and
immersion depth adequate or should they be increased?

The preheat also has to be sufficient at the areas of concern.
Is it according to the prescribed requirement of the flux
manufacturer?
Excess preheat de-activates the flux and insufficient heat does
not vaporize the solvents, making the solder contact more for vaporizing the
solvent and less hole-fill.

Are you also applying enough flux?

The last item is
solderability. Is the base metal under the gold solderable enough for the flux
you are using?

Peter BioccaSenior Market Development EngineerKesterMr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

It's almost always harder to fill open holes than holes with
leads in them. The presence of a lead in a hole helps with heat transfer, and
creates larger capillary force (the force that helps solder wick up the
barrel).

There is no reason,
from a reliability standpoint, that you must solder the entire hole and cover
all of the gold. When you install the jumper, the solder joint must meet the
applicable standards for the class of the assembly, as defined by you (or your
customer). Any additional soldering time or touch-up steps beyond what's
required to meet the standards will increase the heating experienced by the
through-hole and may actually be detrimental to the ultimate reliability of the
assembly.

Fritz ByleProcess EngineerAstronauticsFritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

It's not uncommon for
this effect, and probably more common on all solder joints... but due to
dissimilar colors, you notice it more on the Au plated annular ring.

To minimize the
effect can be a function of the solder mask, and the cleanliness of the PCB
prior to soldering.

If you have control
of the artwork, you might consider the annular ring to be more proportional to
the size of the lead and hole. The size of the hole to lead ratio,
coupled with solder tension can contribute to the solder fillet shape.
Minimize the lead/hole ratio and better match the annular ring and you
can determine what is a value add to the inspection criteria.

The
first question is how many layers of interconnect are there in these holes
typically? When you say exposed gold, do you mean incomplete hole fill and non
top side annular ring wetting? Please confirm that the gold is ENIG. Are you
seeing any other through hole defects or just on these jumper holes? Is the
assembly in a pallet or held by the edge going through the wave solder process.
What is the top side temperature of the board during the wave soldering
process?

Gerard O'BrienPresidentS T and S Testing and AnalysisGerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.

There's a variety of
causes for the solder not to fully wet to pads & hole walls, and none of
them are good. And it gets worse: Non wetting may indicate overall poor bonding
of the solder joint for the areas that actually appear wetted.

Possibilities

The pad/hole wall
surface is contaminated from the PCB Fabricator.

The dwell time
and/or temperature at wave are inadequate.

The jumper wires
themselves are not clean an are introducing contamination.

The gold is the wrong
type (e.g., electro-gold versus ENIG).

Getting Started
Best begin with a
bare PCB. I would evaluate for board cleanliness and correct
gold-type. Then confirm that the jumpers are clean. Proceed to
assembly and validate correct time & temp. at wave, and don't forget wave
purity. Somewhere along the way I feel the root-cause will present itself.

Contamination Forensics
If a bare board is not available, consider
submitting a worse-case assembly to a service bureau specializing in
post-assembly contamination detection & analysis. That science and service
has greatly advanced in the last 5-years.

Best-Guess?
I'm really interested in
the jumpers. PCB contamination is often broad, not selective. Ditto
for gold plating, presuming the entire PCB uses gold as a final finish.

Robert "Bob" Lazzara PresidentCircuit Connect, Inc.Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.

So many answers are possible be it leaded or
Lead free soldering BUT the one that may best fit is lack of Nickel in the
plating and this gives good bottom side wetting but very poor capillary action.

Greg YorkTechnical Sales ManagerBLT Circuit Services LtdGreg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Have
the board fabricated with solder coating in the holes.. and avoid removing gold
from the holes.

Mahendra GandhiSME - PWB TechnologiesNorthrop GrummanMahendra Gandhi has been working in interconnect industry since 1972.

Gold
is the best finish to use to stop oxidization of the boards but is not always
the easiest to put on. This is especially true in through hole plating. You often
find that the gold can be very thin at the shoulder of the hole or at some
point in the barrel, this can lead to poor penetration or incomplete wetting on
the topside. the difficulty is that as the issue is the thickness of the gold
and the potential for oxidization of the nickel surface beneath it you cannot
really inspect before processing to see if it is OK. You can X-section the
failed areas to determine the cause of the defect but that does not solve the
problem.

You
could experiment with other surface finishes if this is allowed to see if they
give better results, tin and silver can sometimes work better but have a
shorter shelf life, the advantage with these finishes is that if there are only
very small areas not soldered then they do not show up as easily as gold. the
disadvantage is that if there are only small areas that are not covered then
they do not show up as easily as gold.

Richard BoyleGlobal Product ChampionHenkel ElectronicsRichard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.

Mr. Roberson is correct. I suspect you are using lead-free solder which does not wet out to the edges of the pad and as a result there may be small amounts of gold still showing which is not a defect condition. As far as the danger of embrittlement, if the PWB plating is ENIG and meets the criteria of IPC 4552, there is little probability you will have any issues with that, as ENIG has less than 5 uinches of gold, not enough to cause any issues.

On the other hand, if insufficient gold is used to seal the nickel underplate from oxidation (which is what you are really soldering to) then the oxidation would also cause poor wetting. However, I doubt that this is happening, because then you would not see any gold in the non-wetted areas.

Richard D. StademAdvanced Engineer/ScientistGeneral DynamicsRichard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

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